An aircraft is typically supported by plural pressurized landing gear struts. Each landing gear strut is designed much like, and incorporates many of the features of, a typical shock absorber. Designs of landing gear incorporate moving components which absorb the impact force of landing. Moving components of an aircraft landing gear shock absorber are commonly telescopic elements. An alternate type of landing gear incorporates a trailing arm design which forms a triangle shape, where the main supporting body of the landing gear is hinged with a trailing arm, and a typical shock absorber functions as the third side of the triangle. The shock absorber of both types of landing gear comprise internal fluids, both hydraulic fluid and compressed nitrogen gas, and function to absorb the vertical descent forces generated when the aircraft lands.
Aircraft have limitations regarding the maximum allowable force the aircraft landing gear and other supporting structures of the aircraft can safely absorb when the aircraft lands. Landing force limitations, which are often related to aircraft vertical velocity (sink-rate) at initial contact with the ground, are a key factor in determining the maximum landing weight for aircraft.
Aircraft routinely depart from an airport with the aircraft weight less than the maximum take-off weight limitation, but greater than the maximum landing weight limitation. During the flight, in-route fuel is burned to reduce the aircraft weight, below the maximum landing weight limitation. Situations often arise where an aircraft has left the departure airport, and the pilot discovers the need to immediately return and land, without the time or opportunity, to burn-off the planned in-route fuel. This causes an overweight landing event. When an overweight landing occurs, the FAA (Federal Aviation Administration) and aircraft manufacturer require the aircraft be removed from service and a manual inspection performed to check for damage of the landing gear and the connection fittings of the landing gear to the aircraft.
Title 14—Part 25, Chapter §25.473 of the current FAA regulations define the assumed maximum vertical velocity at which an aircraft would come into contact with the ground as being ten feet per second (10 fps). The origination of this rule comes from the Civil Aeronautics Board—Civil Air Regulations, Part 4, Chapter §4.24, dated: Nov. 9, 1945. Today an aircraft's maximum landing weight (MLW) limitation is determined by the manufacturer, who must design and demonstrate the structural integrity of the aircraft and landing gear, to allow for the weight of that aircraft to land at maximum landing weight, with a vertical velocity of 10 fps. FAA Regulations assume the aircraft is landing with each of the main landing gear simultaneously touching the ground and the landing force being equally distributed between the two main landing gears. However, cross-wind landings are a common occurrence. In cross-wind situations, the pilot will adjust the lateral angle of the aircraft to lower the wing pointed in the direction of the cross-wind. Lowering this windward wing aides in stabilizing the aircraft against a sudden gust of stronger cross-wind; but also increases the possibility that the aircraft will have an asymmetrical landing gear touch-down. Currently there are no devices certified by the FAA and installed on aircraft to measure and monitor individual landing gear compression rates nor the aircraft descent velocity at initial contact with the ground.
As the aircraft descends towards the runway, the landing gear is extended. Each of the landing gear maintains an internal pre-charge pressure within the shock strut. The pre-charge pressure is a relatively low pressure, which is maintained to insure the landing gear shock absorber component is extended to full strut extension, prior to landing. At full extension, the shock absorber can absorb its maximum amount of landing force. As the landing gear comes into initial contact with the ground, the strut begins to compress, thereby increasing the pressure within the shock absorber. Increases in pressure, beyond the pre-charge pressure, creates additional resistance to the compression rate of the landing gear strut, which helps reduce the vertical velocity of the aircraft.
The FAA requires flight data recorders (FDR) on transport category aircraft. The FDR incorporates multi-axis accelerometers (located at the center of gravity of the aircraft hull) which measure various shock loads that become evident during an abrupt landing event. The accelerometer data is usually not available unless an accident has occurred, and the FDR is removed from the aircraft, the data downloaded, and then analyzed. Assuming one might attempt to determine landing gear compression rate from the FDR data, the information would be merely calculations from measurements taken not at the respective landing gear locations of the aircraft, but along the center-line of the aircraft. The FDR calculations would not be associated with the compression rate of any respective landing gear strut, but rather any changes in acceleration for the aircraft hull as a whole.
Research of prior art identifies numerous systems which measure whole aircraft descent velocity. Though it is advantageous for pilots to know the average descent velocity or sink-rate of the aircraft while approaching a runway for landing, the actual descent velocity can vary drastically due to non-pilot actions including such factors as varying wind conditions. The descent velocity of the whole aircraft hull the does not necessarily indicate the compression rate of any respective landing gear strut as it comes into initial contact with the ground.
Prior art to determine aircraft descent velocity is well documented. Reference is made to U.S. Pat. No. 3,712,1228—Harris; U.S. Pat. No. 6,012,001—Scully, and U.S. Pat. No. 4,979,154—Brodeur. These and other patents describing similar but subtly different techniques teaching the use of various range-finder devices, attached to the aircraft hull, which measure the distance between the aircraft hull and the ground, as well as the rate of change of those measurements. The prior art does not measure the compression rate of each respective landing gear, as they come into contact with the ground; nor do they take the approach of using more advanced, short-range, high accuracy, targeted sonic or laser measuring devices, to measure the actual compression rate of the landing gear strut, by measuring closure rates between the relatively short distance from a targeted point of the lower moving portion of the landing gear strut as compared to the fixed portion of the upper strut, or the under surface of the aircraft hull. Reducing the range of the measurement increases the accuracy of the measurement.
As an aircraft approaches a runway for landing, if the pilot properly flares the aircraft, the sink-rate of the aircraft will dramatically reduce, just above the runway surface. During the aircraft flare procedure, a cushion of air is developed by the downward force of air generated by the wing coming near the ground surface. This cushion of air is often referred to as “ground effect” and will substantially reduce the descent velocity of the aircraft. In ground effect, the aircraft is reaching a stall situation which reduces the lifting force generated by the wings. Aircraft wing oscillation can occur, where the aircraft wings flutter from side to side. This is another situation where an asymmetrical landing gear touch-down will occur. Aircraft sink-rate, measured with accelerometers along the centerline of the aircraft, will not detect wing oscillation and will not determine the initial compression rate experienced by each individual land gear, when the aircraft comes into initial contact with the ground.
Additional search of prior art relating to landing gear identified U.S. Pat. No. 2,587,628—King, which teaches an apparatus for testing “yieldable load carrying structures” such as aircraft landing gear. King teaches monitoring the rate of deceleration of the mass supported by the landing gear and the effects on other connected landing gear elements. King teaches the relationship between the telescopic compression of the landing gear, as compared to shear deflection to other structural members of that same landing gear. King teaches apparatus used as a tool to determine the effective change in the fatigue life limitations of a particular landing gear structural component, by tracking the rate of change in force applied to the shock absorbing components attached to said fatigue life limited structural component. King does not teach landing gear strut rate of compression, as related to aircraft sink rate at initial contact with the ground.
U.S. Pat. No. 3,517,550—Leventhal, teaches the relationship of comparing internal strut pressure increases, as related to the rate of landing gear strut compression, thereby determining the rate of change in descent velocity. Though the approach may appear valid, it is subject to error by its inability to verify, at any given landing event, the exact proportion of gas volume in relation to hydraulic oil volume, within the landing gear strut.
U.S. Pat. No. 6,128,951—Nance, teaches the measuring of strut pressure within each landing gear strut, as well as determining the current proportion or ratio of gas to hydraulic oil within each respective landing gear strut. Internal strut pressure, compared to strut extension, is not a linear relationship. Commonly aircraft maintenance technicians observed landing gear struts which appear near deflated, due to hydraulic oil having escaped through the strut seals. Mistakenly assuming the landing gear has lost nitrogen gas, the technician adds additional gas to the strut, thus the landing gear strut is now over-charged with gas. The now changed and unknown volume of gas being compressible and that variance in volume of gas as compared to the unknown volume of non-compressible hydraulic oil having changed, would thereby vary the compression rate of the landing gear strut and generate errors in the velocity calculation. Also, pressure within a landing gear strut is contained by the friction of the landing gear strut seals.
U.S. Pat. No. 5,214,586—Nance teaches distortions in landing gear strut pressure measurements caused by landing gear strut seal friction. Landing gear strut seal friction can distort internal strut pressure measurements by as much as 5% of the applied weight. Attempts to determine initial aircraft touch-down velocity at the landing gear strut would be subject to errors caused by the friction of the strut seals distorting pressure measurements and delaying any increases in internal landing gear strut pressure. These delays in any increase in strut pressure due to strut seal friction would distort the accuracy of a direct comparison of rate of internal pressure increases to strut compression.
U.S. Pat. Nos. 7,193,530; 7,274,309; 7,274,310—Nance teach the measurement of the rate of compression of landing gear strut by a different means than that of the new invention described in this application. The prior art of Nance teaches the use of mechanical rotation sensors to measure rotation rates of rotating elements (scissor links) of the telescopic landing gear, then using geometry to determine the rate of landing gear strut compression; combined with the pressure sensors monitoring internal strut pressure increases, as a cross-check function to increase confidence in the accuracy of the mechanical measurement.